Apartment complexes built into huge repurposed natural gas tanks.

This grand manse has been transformed into a museum devoted to one of the founders of Fantastic Realism.

This cubist Vienna church building looks like some sort of alien stonehenge.

Located right outside of Vienna, visitors can take in expansive views of an ethereal forest.

The topiary maze at the Habsburg hunting lodge.

Along the edge of this fountain are 365 smaller fountains that represent the days of the year.

Vienna's underworld of tunnels and subterranean rivers made an iconic appearance in a 1949 Orson Welles film.

This series of Baroque passageways in Vienna's cultural center mix old architecture with modern art.

The housing complex served as an anti-fascist fortress during the Austrian Civil War.

One of the most photographed buildings in a city known for its Modernista architecture is an abandoned squat.

This semi-secret storybook hedge maze is also the oldest garden in the city.

Bask in the 19th century grandeur of an eccentric Freemason millionaire's private gardens.

A former steam-powered mill from the 1800s is now an industrial park with a giant steel dragon slide.

An architect transformed this abandoned cement factory into his castle-like workshop.

The abandoned ruins of a beautiful medieval castle just outside Barcelona.

A state-of-the-art medical facility from the Imperial era is now a derelict ruin.

The world's first motorway was used as a racetrack until the late 1990s.

The skeletal frame of a decommissioned gas-pumping station offers breathtaking views of greater Berlin.

Cold war icon and onetime "hole" in the iron curtain.

Residents of Berlin take their coffee and cake in one of the city’s most restful spaces: a cemetery.

These former Nazi anti-aircraft towers offer a singular view of the north of Berlin in the summer, and a sanctuary for bats in winter.

Berlin's old airport.

Floating swimming pool within a moored vessel.

Super-heavy Nazi architectural experiment.